1. Field of the Invention
The present invention relates to an apparatus for measuring the flow-pressure characteristics of a gas passing through a product sample with two faces, comprising a sample holder adapted so that one of the faces of the sample is subjected to the atmospheric pressure, a pump for applying a sub-atmospheric pressure on the other face of the sample, a sensor for measuring the pressure differential, a sensor for measuring the gas flow through the sample which results from said pressure differential, a reference signal generator and means for comparing the output of one of the two sensors and the output of the reference signal generator.
Such an apparatus is in particular used for measuring the permeability to air of materials used as cigarette paper, or sleeving for filters, in compliance for example with the international standard ISO 2965 or with the French standard NF V37-010. The permeability is the ratio of the air flow (volume per unit of time) per unit of surface of the sample to the pressure difference through this sample.
2. Description of the Prior Art
The British patent application No. 2 094 986A describes an apparatus of the above type in which, however, neither reference signal generator nor means for comparing the output of one of the sensors and the output of the reference signal generator are provided. In this apparatus, means are provided for varying the air flow through the sample, formed in particular by a battery of constant flow devices mounted in parallel, and connecting the sample holder to the pump. In series with each device is connected a valve and the nominal values of the flows likely to pass through each of the branches are in geometric progression: the first device, when it is operating, has necessarily passing therethrough a flow of 1 liter per minute, the second a flow of two liters per minute, the third a flow of four liters per minute and so on.
Now, in order to make measurements in compliance with the standard ISO 2965, each permeability measurement must be made under specified pressure differential conditions, for example 0.25 and 1 kilopascal.
The quantification of the flow rates which may be achieved with the known apparatus involve a quantification of the pressure differential which does not allow, except by chance, to repeat materially the preceeding pressure differential conditions. It is therefore necessary, with this system, to seek combinations of constant flow devices such that the corresponding measured pressure differential bracket the specified value as closely as possible. The permeability for this specified value is then obtained by an interpolation calculation from the measured values.
The result is a long and tiresome measuring operation, the result of which is impaired through an error related to the interpolation.
To overcome these drawbacks, the U.S. Pat. No. 4,311,037 provides a reference signal generator, means for comparing the output of one of the two sensors with the output of the reference signal generator and a pump with variable speed of rotation whose rotational speed is controlled by the output of the comparison means.
Thus, when such an apparatus is adapted, for example, so that the output of the sensor measuring the pressure differential is applied to the comparison means, it allows a constant pressure differential to be obtained permanently, whose value is equal to the specified value, by means of the reference signal generator.
However, such an apparatus can only respond slowly to relatively rapid variations at the output of the reference generator, because the regulation for obtaining a given flow, or a given pressure differential, is obtained by varying the rotational speed of the pump. This results in inaccuracies in the measurements when they must be made rapidly, or risks of having the whole apparatus self oscillating.
Furthermore, with the apparatus described in the U.S. Pat. No. 4,311,037, the testing of a large number of separate samples is long and tiresome, for it is necessary to stop the pump for each sample change and then to wait for a fairly long time until the pressure differential is reestablished.
Finally, in this apparatus, the measurement of the flow passing through the sample uses a venturi, i.e. a restriction in a pipe. Now, such a device has a non linear flow-pressure characteristic, which limits the use of the apparatus to a relatively narrow range of flow rates if it is desired to avoid tiresome corrections.